This invention relates to tertiary oil recovery.
It has long been known that after primary oil recovery is exhausted, additional oil can be obtained by secondary recovery wherein a fluid is injected into certain wells and oil is produced from other wells. Various techniques such as a five-spot system wherein fluid is injected into a central well and oil is produced from four surrounding wells, or vice versa, are well known. Also known are sequential systems employing seven spots as described in U.S. Pat. No. 3,120,870.
However, during the injection of preflush prior to beginning a tertiary oil recovery operation, no significant amount of oil is being produced. Hence it is essential to keep the preflush injection as brief as possible.
After secondary recovery is complete and little or no more oil can be produced thereby, there is still a great deal of oil remaining in the ground. It is known to remove part of this unrecovered oil by tertiary recovery wherein a microemulsion bank or a surfactant solution is injected through the same wells used in the secondary recovery. As yet, tertiary recovery has achieved only limited commercial success because of the cost of the ingredients; particularly in the microemulsion flooding, relatively expensive alcohols and petroleum sulfonates are required and there is a tendency for the petroleum sulfonate to be lost due to adsorption, or destruction by divalent cations in the residual formation water. Thus, in order to get the most efficient utilization of the expensive surfactants, it is necessary to displace the residual formation water which may contain a high TDS (greater than 50,000 parts per million total dissolved solids) and/or high concentration of divalent cations, with a medium compatible with the tertiary recovery fluid. The divalent cations form oil soluble salts with the sulfonate and these salts are not effective in oil displacement.
Unfortunately, it has been found that the use of the techniques developed for injecting brine in secondary recovery operations are not very efficient for injecting the preflush to displace the residual formation water. This is because the preflush tends to form cusps and break through to the production wells after about 70 to 72% of the pore volume has been flushed. This breakthrough of the preflush fluid directly from the injection well to the production well makes this technique uneconomical. Of course, the same thing happens in secondary recovery by means of waterflooding; however, this can be tolerated since, after breakthrough occurs, continued injection of the brine results in the production of a mixture of oil and drive water until most of the pore volume has been swept, and the continuous recovery of an increment of oil makes the procedure economically feasible.